Preheat oven to 350F. Line 3, 8-inch cake pans with parchment paper; recipe will also work well using 2, 9-inch cake pans.
Using a hand mixer or a stand mixer, cream together the sugar and butter until creamy, about 4 minutes. Add the egg whites, vanilla and champagne extract and mix until mixture is light and airy, about 3 minutes. Pour in the sparkling wine and milk but do not mix. In a separate bowl, combine the dry ingredients: flour, salt and baking powder. Sift the dry ingredients into the cake batter and mix the flour just until well combined.
Divide the cake batter between the prepared pans. I recommend using a kitchen scale to accurately divide the batter. Bake the cake layers in preheated oven for 35-40 minutes, or until the tops are completely set. Remove from oven and place cake rounds onto a cool rack to cool completely.
Meanwhile, prepare the frosting and filling. For the filling, simply mash together 1 cup of fresh strawberries (or thawed, previously frozen berries); set aside. For the buttercream, whip the softened butter on high speed for 5 to 7 minutes until light and fluffy. Add the 1/3 cup strawberry puree (I used a food processor to make the puree) and the strawberry extract. I used a strawberry emulsion, which already had food coloring. Otherwise, also add a few drops of red food coloring. Add the citric acid, or if unavailable, substitute with 2 tablespoons of lemon juice. Whisk buttercream until ingredients are well combined. Add the confectioner's sugar and continue beating the buttercream for another 5 to 7 minutes until light and fluffy.
To assemble the cake: level the cake layers using a long serrated knife. Spread half of the mashed strawberries onto the first layer, followed by buttercream and second cake layer. Use the remaining strawberries on the second layer. Frost the top and sides of cake and use a flat spatula to even off the top and sides. Place into refrigerator to set before garnishing, about 20 minutes.
Optional: transfer any remaining buttercream into a pastry bag and add dollops of frosting on top. Can also top with chocolate covered strawberries; strawberries will last for 24 hours. If making this cake ahead of time, wait to add the strawberries until serving. Keep cake refrigerated and take out to room temperature 30 minutes before serving.